Physicists find surprising ‘liquid-like’ particle interactions in Large Hadron Collider

Rice undergraduate student Benjamin Tran, graduate student Michael Northup, postdoctoral student Maxime Guilbaud and graduate students Zhenyu Chen and Zhoudunming Tu were part of the Rice team of physicists on the Large Hadron Collider’s Compact Muon Solenoid experiment that co-authored a paper describing the unexpected particle interactions from proton and lead-nuclei collisions. Credit: Zhoudunming Tu

by Don Lincoln

Three years ago, Rice physicists and their colleagues on the Large Hadron Collider’s (LHC’s) Compact Muon Solenoid (CMS) experiment stumbled on an unexpected phenomenon. Physicists smashed protons into lead nuclei at nearly the speed of light, which caused hundreds of particles to erupt from these collisions. But that wasn’t the surprise. What was surprising is where these particles went: Rather than spreading out evenly in all directions, the particles coming out of the collisions preferentially lined up in a specific direction.

Now, the Rice team has co-authored a paper that describes the unexpected particle interactions from these proton and lead-nuclei collisions.

Particle detectors are shaped a little like a soup can. In these kinds of collisions, there is a tendency for particles to amass in a line along the axis of the can known as a “ridge.” Up until now, physicists understood a lot about what happens when a pair of protons or a pair of lead nuclei collide, but not a lot about what happens when a proton hits a lead nucleus: Would the hot nuclear matter coming out of the collision act like protons colliding, in which the post-collision particles coast along without feeling the effect of their neighbors? Or would the particles coming out of proton and lead collisions act in a more collective, liquid-like way as in lead-nuclei collisions?

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